Pulmonary oedema is a clinical condition that is caused by a number of different diseases such as heart failure or kidney diseases. It consists of an accumulation of fluid in the interstitial and alveolar space of the lung following increased blood pressure in the pulmonary capillary vessels that lead to leakage of water from the blood to the lung space. This condition has a progressive nature and clinical signs of pulmonary oedema occur late, typically when the lung fluid has increased at least six-fold from the initial stage of interstitial oedema. This means that pulmonary oedema is often not detected early, and necessary treatment for the patient is delayed.
Existing clinical methods to detect pulmonary oedema are invasive and expensive, such as chest radiography or pulmonary capillary wedge-pressure measurements. Pre-clinical detection and monitoring of lung fluid volume would enhance medical treatment and potentially reduce medical costs resulting from hospitalization of a patient with pulmonary oedema.
Bio-impedance spectroscopy (BIS) is a low-cost and non-invasive technique for detecting pulmonary oedema. The principle underlying this technique is the fact that the electrical impedance (resistance and reactance) of the biological tissue is directly linked to the hydration and water content of the tissue, namely intra-cellular and extra-cellular water. Therefore, measurements of the electrical properties of the tissue can indicate the amount of fluid present that part of the body. The BIS method has been used to determine the total amount of water in the body and body composition (i.e. fat and fat free mass). Recently, BIS has been used to monitor the accumulation of fluid as an indicator for pulmonary oedema using whole-body, intra-thoracic or trans-thoracic measurement systems.
Intra-thoracic and trans-thoracic BIS provide a more sensitive and direct measure of pulmonary fluid content than whole-body techniques, which detect fluid accumulation in both peripheral and pulmonary regions. However, thoracic BIS measurements can only currently be used to generate qualitative assessments of changes in lung fluid content over time, which can indicate the onset of pulmonary oedema, and it is not yet possible to use these measurements to quantify the amount of fluid in the lungs nor to quantify the change in lung fluid content.
Therefore, there is a need for an improved method and apparatus for estimating the fluid content of the lungs from bio-impedance measurements that can provide quantitative indications of the amount of fluid in the lungs and/or the change in lung fluid content. Such a method and apparatus could be used in a monitoring system to detect the presence and progression of pulmonary oedema, as well as monitoring improvements in the patient's condition as a result of receiving treatment.